Polynucleotides encoding phosphoribosylanthranilate isomerase

ABSTRACT

This invention relates to isolated nucleic acid fragments encoding phosphoribosylanthranilate isomerases, a tryptophan biosynthetic enzyme. The invention also relates to the construction of a chimeric gene comprising the nucleic acid fragment, in sense or antisense orientation, wherein expression of the chimeric gene results in production of altered levels of the phosphoribosylanthranilate isomerase in a transformed host cell.

This application is a divisional of U.S. application Ser. No. 09/647,224, filed Jul. 16, 2001, now U.S. Pat. No. 6,482,631, which is the National Stage Application, according to 35 U.S.C. 371, of International Application No. PCT/US99/06582, filed Mar. 22, 1999, which claims the benefit of U.S. Provisional Application No. 60/079,386, filed Mar. 26, 1998.

FIELD OF THE INVENTION

This invention is in the field of plant molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding enzymes involved in tryptophan biosynthesis in plants and seeds.

BACKGROUND OF THE INVENTION

Many vertebrates, including man, lack the ability to manufacture a number of amino acids and therefore require these amino acids preformed in their diet. These are called essential amino acids. Plants are able to synthesize all twenty amino acids and serve as the ultimate source of the essential amino acids for humans and animals. Thus, the ability to manipulate the production and accumulation of the essential amino acids in plants is of considerable importance and value. Furthermore, the inability of animals to synthesize these amino acids provides a useful distinction between animal and plant cellular metabolism. This can be exploited for the discovery of herbicidal chemical compounds that target enzymes in the plant biosynthetic pathways of the essential amino acids and thus have low toxicity to animals.

Tryptophan is an essential amino acid. In plants, the biosynthesis of tryptophan from chorismic acid (see FIGS. 1A and 1B) requires five enzymatic steps catalyzed by anthranilate synthase (EC 4.1.3.27), anthranilate phosphoribosyl-transferase (EC 2.4.2.18), phosphoribosylanthranilate isomerase (EC 5.3.1.24), indole-3-glycerol phosphate synthase (EC 4.1.1.48) and tryptophan synthase (EC 4.2.1.20). The tryptophan pathway leads to the biosynthesis of many secondary metabolites including the hormone indole-3-acetic acid, antimicrobial phytoalexins, alkaloids and glucosinolates. Anthranilate phosphoribosyl-transferase is encoded by the PAT1 locus in Arabidopsis thaliana and the trpD locus in bacteria. Anthranilate phosphoribosyltransferase catalyzes the second step in tryptophan biosynthesis from chorismate forming 5-phosphoribosylanthranilate from anthranilate. Arabidopsis mutants in this gene are blue fluorescent under UV light due to accumulation of anthranilate compounds. Analysis of Arabidopsis plants expressing translational fusions of betaglucuronidase and different sections of the PAT1 gene indicates that the entire plastid transit peptide and the first two introns of PAT 1 are required for efficient transcription and translation (Rose, A. B. and Last, R. L. (1997) Plant J 11:455–464). Anthranilate phosphoribosyltransferase purifies from Saccharomyces cerevisiae as a dimer (Hommel, U. et al. (1989) Eur J Biochem 180:33–40).

Phosphoribosylanthranilate isomerase catalyzes the third step in tryptophan biosynthesis from chorismate forming 1-(O-carboxyphenylamino)-1-deoxyribulose-5-phosphate. Three nonallelic genes encode phosphoribosylanthranilate isomerase in Arabidopsis thaliana. All three alleles contain a plastid transit peptide at their N-terminus, are over 90% identical and are flanked by nearly identical 350 nucleotide repeats (Li, J. Y. et al. (1995) Plant Cell 7:47–461).

Indole-3-glycerol phosphate synthase catalyzes the fifth step in tryptophan biosynthesis from chorismate producing indole-glycerol phosphate from 1-(2-carboxyphenylamino)-1-deoxyribulose 5′-phosphate. Mutation of seven invariant polar residues in the active site of the enzyme from Escherichia coli have allowed the identification of catalytically essential residues. Random saturation mutagenesis indicates that K114, E163, E53 and N184 are located in the active site of the enzyme (Darimont, B. et al. (1998) Protein Sci 7:1221–1232).

Few of the genes encoding enzymes from the tryptophan pathway in corn, soybeans, rice and wheat, have been isolated and sequenced. For example, no corn, soybean, rice or wheat genes have been reported for anthranilate phosphoribosyltransferase, phosphoribosylanthranilate isomerase or indole-3-glycerol phosphate synthase. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate studies to better understand cellular biosynthetic pathways, provide genetic tools for the manipulation of those pathways, provide a means to evaluate chemical compounds for their ability to inhibit the activity of enzymes in the tryptophan biosynthetic pathway.

SUMMARY OF THE INVENTION

The instant invention relates to isolated nucleic acid fragments encoding tryptophan biosynthetic enzymes. Specifically, this invention concerns an isolated nucleic acid fragment encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase. In addition, this invention relates to a nucleic acid fragment that is complementary to the nucleic acid fragment encoding anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase.

An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a tryptophan biosynthetic enzyme selected from the group consisting of anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase and phosphoribosylanthranilate isomerase.

In another embodiment, the instant invention relates to a chimeric gene encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase, or to a chimeric gene that comprises a nucleic acid fragment that is complementary to a nucleic acid fragment encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of levels of the encoded protein in a transformed host cell that is altered (i.e., increased or decreased) from the level produced in an untransferred host cell.

In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase, operably linked to suitable regulatory sequences. Expression of the chimeric gene results in production of altered levels of the encoded protein in the transformed host cell. The transformed host cell can be of eukaryotic or prokaryotic origin, and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and seeds derived from such transformed plants.

An additional embodiment of the instant invention concerns a method of altering the level of expression of an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding an anthranilate phosphoribosil-transferase, an indole-3-glycerol phosphate synthase or a phosphoribosyl-anthranilate isomerase; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosyl-anthranilate isomerase in the transformed host cell.

An addition embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or a substantial portion of an amino acid sequence encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase.

A further embodiment of the instant invention is a method for evaluating at least one compound for its ability to inhibit the activity of an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase, the method comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase, operably linked to suitable regulatory sequences; (b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase in the transformed host cell; (c) optionally purifying the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase expressed by the transformed host cell; (d) treating the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase with a compound to be tested; and (e) comparing the activity of the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase that has been treated with a test compound to the activity of an untreated anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosyl-anthranilate isomerase, thereby selecting compounds with potential for inhibitory activity.

BRIEF DESCRIPTION OF THE DRAWINGS AND SEQUENCE DESCRIPTIONS

The invention can be more fully understood from the following detailed description and the accompanying drawings and Sequence Listing which form a part of this application.

FIGS. 1A and 1B depicts biochemical pathway for the production of tryptophan from chorismate.

FIGS. 2A and 2B shows an alignment of the amino acid sequences from Arabidopsis thaliana indole-3-glycerol phosphate synthase (SEQ ID NO:25), the instant corn indole-3-glycerol phosphate synthase (contig of p0128.cpicq73r, p0041.crtav17rb, p0002.cgevb40r, p0091.cmarc86r, cr1n.pk0121.b7 and chp2.pk0003.c4; SEQ ID NO:10) and the instant soybean indole-3-glycerol phosphate synthase (sdp2c.pk001.f3; SEQ ID NO:14). Amino acid which are identical among all sequences are indicated with an asterisk (*) above the alignment. Dashes are used by the program to maximize alignment of the sequences.

FIGS. 3A and 3B shows an alignment of the amino acid sequences from Arabidopsis thaliana phosphoribosylanthranilate sisomerase (SEQ ID NO:26), the instant corn phosphoribosylanthranilate isomerase (ceb1.pk0026.d2; SEQ ID NO:20) and the instant wheat phosphoribosylanthranilate isomerase (wr1.pk0127.e10; SEQ ID NO:24). Amino acid which are identical among all sequences are indicated with an asterisk (*) above the alignment. Dashes are used by the program to maximize alignment of the sequences.

The following sequence descriptions and Sequence Listing attached hereto comply with the rules governing nucleotide and/or amino acid sequence disclosures in patent applications as set forth in 37 C.F.R. §1.821–1.825.

SEQ ID NO:1 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA insert in clones p0037.crwan82r, p0022.cglne15r, cbn10.pk0045.b8, p0109.cdadc66r, cr1n.pk0145.g6, cco1.pk0002.f11, cs1.pk0056.d10, p0103.ciaag55r and p0104.cabbg36r encoding an entire corn anthranilate phosphoribosyltransferase.

SEQ ID NO:2 is the deduced amino acid sequence of an entire corn anthranilate phosphoribosyltransferase derived from the nucleotide sequence of SEQ ID NO:1.

SEQ ID NO:3 is the nucleotide sequence comprising the entire cDNA insert in clone ssm.pk0059.f8 encoding the C-terminal half of a soybean anthranilate phosphoribosyltransferase.

SEQ ID NO:4 is the deduced amino acid sequence of the C-terminal half of a soybean anthranilate phosphoribosyl-transferase derived from the nucleotide sequence of SEQ ID NO:3.

SEQ ID NO:5 is the nucleotide sequence comprising the entire cDNA insert in clone wre1.pk0003.b12 encoding the C-terminal half of a wheat anthranilate phosphoribosyltransferase.

SEQ ID NO:6 is the deduced amino acid sequence of the C-terminal half of a wheat anthranilate phosphoribosyl-transferase derived from the nucleotide sequence of SEQ ID NO:5.

SEQ ID NO:7 is the nucleotide sequence comprising the entire cDNA insert in clone cen3n.pk0147.h5 encoding a substantial portion of a corn indole-3-glycerol phosphate synthase.

SEQ ID NO:8 is the deduced amino acid sequence of a substantial portion of a corn indole-3-glycerol phosphate synthase derived from the nucleotide sequence of SEQ ID NO:7.

SEQ ID NO:9 is the nucleotide sequence comprising the contig assembled from the cDNA insert in clones p0128.cpicq73r, p0041.crtav17rb, p0002.cgevb40r, p0091.cmarc86r and the entire cDNA insert in clones crln.pk0121.b7 and chp2.pk0003.c4 encoding an entire corn indole-3-glycerol phosphate synthase.

SEQ ID NO:10 is the deduced amino acid sequence of an entire corn indole-3-glycerol phosphate synthase derived from the nucleotide sequence of SEQ ID NO:9.

SEQ ID NO:11 is the nucleotide sequence comprising the entire cDNA insert in clone r10n.pk0021.f11 encoding a portion of a rice indole-3-glycerol phosphate synthase.

SEQ ID NO:12 is the deduced amino acid sequence of a portion of a rice indole-3-glycerol phosphate synthase derived from the nucleotide sequence of SEQ ID NO:11.

SEQ ID NO:13 is the nucleotide sequence comprising the entire cDNA insert in clone sdp2c.pk001.f3 encoding an entire soybean indole-3-glycerol phosphate synthase.

SEQ ID NO:14 is the deduced amino acid sequence of an entire soybean indole-3-glycerol phosphate synthase derived from the nucleotide sequence of SEQ ID NO:13.

SEQ ID NO:15 is the nucleotide sequence comprising the entire cDNA insert in clone wre1n.pk0075.b10 encoding a substantial portion of a wheat indole-3-glycerol phosphate synthase.

SEQ ID NO:16 is the deduced amino acid sequence of a substantial portion of a wheat indole-3-glycerol phosphate synthase derived from the nucleotide sequence of SEQ ID NO:15.

SEQ ID NO:17 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA insert in clones cco1n.pk0030.b11, p0068.clsaa67r and p0099.ctbai70r encoding the N-terminal half of a corn phosphoribosylanthranilate isomerase.

SEQ ID NO:18 is the deduced amino acid sequence of the N-terminal half of a corn phosphoribosylanthranilate isomerase derived from the nucleotide sequence of SEQ ID NO:17.

SEQ ID NO:19 is the nucleotide sequence comprising the entire cDNA insert in clone ceb1.pk0026.d2 encoding an entire corn phosphoribosylanthranilate isomerase.

SEQ ID NO:20 is the deduced amino acid sequence of an entire corn phosphoribosylanthranilate isomerase derived from the nucleotide sequence of SEQ ID NO:19.

SEQ ID NO:21 is the nucleotide sequence comprising a portion of the cDNA insert in clone rsr9n.pk001.g2 encoding the C-terminal half of a rice phosphoribosylanthranilate isomerase.

SEQ ID NO:22 is the deduced amino acid sequence of the C-terminal half of a rice phosphoribosylanthranilate isomerase derived from the nucleotide sequence of SEQ ID NO:21.

SEQ ID NO:23 is the nucleotide sequence comprising the entire cDNA insert in clone wr1.pk0127.e10 encoding an entire wheat phosphoribosyl-anthranilate isomerase.

SEQ ID NO:24 is the deduced amino acid sequence of an entire wheat phosphoribosylanthranilate isomerase derived from the nucleotide sequence of SEQ ID NO:23.

SEQ ID NO:25 is the amino acid sequence of an Arabidopsis thaliana indole-3-glycerol phosphate synthase having an NCBI General Identifier No. 1351303.

SEQ ID NO:26 is the amino acid sequence of a Arabidopsis thaliana phosphoribosylanthranilate isomerase having an NCBI General Identifier No. 619749.

SEQ ID NO:27 is the nucleotide sequence of the PCR primer pIGSs-F1.

SEQ ID NO:28 is the nucleotide sequence of the PCR primer pIGSs-F2.

SEQ ID NO:29 is the nucleotide sequence of the PCR primer pIGSs-M1.

SEQ ID NO:30 is the nucleotide sequence of the PCR primer pIGSs-M2.

SEQ ID NO:31 is the nucleotide sequence of the PCR primer pSK-B1.

SEQ ID NO:32 is the nucleotide sequence of the PCR primer pSK-B2.

The Sequence Listing contains the one letter code for nucleotide sequence characters and the three letter codes for amino acids as defined in conformity with the IUPAC-IUBMB standards described in Nucleic Acids Research 13:3021–3030 (1985) and in the Biochemical Journal 219 (No. 2):345–373 (1984) which are herein incorporated by reference. The symbols and format used for nucleotide and amino acid sequence data comply with the rules set forth in 37 C.F.R. §1.822.

DETAILED DESCRIPTION OF THE INVENTION

In the context of this disclosure, a number of terms shall be utilized. As used herein, an “isolated nucleic acid fragment” is a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases. An isolated nucleic acid fragment in the form of a polymer of DNA may be comprised of one or more segments of cDNA, genomic DNA or synthetic DNA. As used herein, “contig” refers to an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence. For example, several DNA sequences can be compared and aligned to identify common or overlapping regions. The individual sequences can then be assembled into a single contiguous nucleotide sequence.

As used herein, “substantially similar” refers to nucleic acid fragments wherein changes in one or more nucleotide bases results in substitution of one or more amino acids, but do not affect the functional properties of the protein encoded by the DNA sequence. “Substantially similar” also refers to nucleic acid fragments wherein changes in one or more nucleotide bases does not affect the ability of the nucleic acid fragment to mediate alteration of gene expression by antisense or co-suppression technology. “Substantially similar” also refers to modifications of the nucleic acid fragments of the instant invention such as deletion or insertion of one or more nucleotides that do not substantially affect the functional properties of the resulting transcript vis-à-vis the ability to mediate alteration of gene expression by antisense or co-suppression technology or alteration of the functional properties of the resulting protein molecule. It is therefore understood that the invention encompasses more than the specific exemplary sequences.

For example, it is well known in the art that antisense suppression and co-suppression of gene expression may be accomplished using nucleic acid fragments representing less than the entire coding region of a gene, and by nucleic acid fragments that do not share 100% sequence identity with the gene to be suppressed. Moreover, alterations in a gene which result in the production of a chemically equivalent amino acid at a given site, but do not effect the functional properties of the encoded protein, are well known in the art. Thus, a codon for the amino acid alanine, a hydrophobic amino acid, may be substituted by a codon encoding another less hydrophobic residue, such as glycine, or a more hydrophobic residue, such as valine, leucine, or isoleucine. Similarly, changes which result in substitution of one negatively charged residue for another, such as aspartic acid for glutamic acid, or one positively charged residue for another, such as lysine for arginine, can also be expected to produce a functionally equivalent product. Nucleotide changes which result in alteration of the N-terminal and C-terminal portions of the protein molecule would also not be expected to alter the activity of the protein. Each of the proposed modifications is well within the routine skill in the art, as is determination of retention of biological activity of the encoded products. Moreover, the skilled artisan recognizes that substantially similar nucleic acid sequences encompassed by this invention are also defined by their ability to hybridize, under stringent conditions (0.1×SSC, 0.1% SDS, 65° C.), with the sequences exemplified herein. Substantially similar nucleic acid fragments of the instant invention may also be characterized by the percent identity of the amino acid sequences that they encode to the amino acid sequences disclosed herein, as determined by algorithms commonly employed by those skilled in this art. Preferred are those nucleic acid fragments whose nucleotide sequences encode amino acid sequences that are greater than 80% identical to the amino acid sequences reported herein. More preferred nucleic acid fragments encode amino acid sequences that are 90% identical to the amino acid sequences reported herein. Most preferred are nucleic acid fragments that encode amino acid sequences that are 95% identical to the amino acid sequences reported herein. Sequence alignments and percent similarity calculations were performed using the Megalign program of the LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences was performed using the Clustal method of alignment (Higgins, D. G. and Sharp, P. M. (1989) CABIOS. 5:151–153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise alignments using the Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5.

A “substantial portion” of an amino acid or nucleotide sequence comprises enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to afford putative identification of that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., (1993) J. Mol. Biol. 215:403–410). In general, a sequence often or more contiguous amino acids or thirty or more nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene specific oligonucleotide probes comprising 20–30 contiguous nucleotides may be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oligonucleotides of 12–15 bases may be used as amplification primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a “substantial portion” of a nucleotide sequence comprises enough of the sequence to afford specific identification and/or isolation of a nucleic acid fragment comprising the sequence. The instant specification teaches partial or complete amino acid and nucleotide sequences encoding one or more particular plant proteins. The skilled artisan, having the benefit of the sequences as reported herein, may now use all or a substantial portion of the disclosed sequences for purposes known to those skilled in this art. Accordingly, the instant invention comprises the complete sequences as reported in the accompanying Sequence Listing, as well as substantial portions of those sequences as defined above.

“Codon degeneracy” refers to divergence in the genetic code permitting variation of the nucleotide sequence without effecting the amino acid sequence of an encoded polypeptide. Accordingly, the instant invention relates to any nucleic acid fragment that encodes all or a substantial portion of the amino acid sequence encoding the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase proteins as set forth in SEQ ID NOs:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22 and 24. The skilled artisan is well aware of the “codon-bias” exhibited by a specific host cell in usage of nucleotide codons to specify a given amino acid. Therefore, when synthesizing a gene for improved expression in a host cell, it is desirable to design the gene such that its frequency of codon usage approaches the frequency of preferred codon usage of the host cell.

“Synthetic genes” can be assembled from oligonucleotide building blocks that are chemically synthesized using procedures known to those skilled in the art. These building blocks are ligated and annealed to form gene segments which are then enzymatically assembled to construct the entire gene. “Chemically synthesized”, as related to a sequence of DNA, means that the component nucleotides were assembled in vitro. Manual chemical synthesis of DNA may be accomplished using well established procedures, or automated chemical synthesis can be performed using one of a number of commercially available machines. Accordingly, the genes can be tailored for optimal gene expression based on optimization of nucleotide sequence to reflect the codon bias of the host cell. The skilled artisan appreciates the likelihood of successful gene expression if codon usage is biased towards those codons favored by the host. Determination of preferred codons can be based on a survey of genes derived from the host cell where sequence information is available.

“Gene” refers to a nucleic acid fragment that expresses a specific protein, including regulatory sequences preceding (5′ non-coding sequences) and following (3′ non-coding sequences) the coding sequence. “Native gene” refers to a gene as found in nature with its own regulatory sequences. “Chimeric gene” refers any gene that is not a native gene, comprising regulatory and coding sequences that are not found together in nature. Accordingly, a chimeric gene may comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. “Endogenous gene” refers to a native gene in its natural location in the genome of an organism. A “foreign” gene refers to a gene not normally found in the host organism, but that is introduced into the host organism by gene transfer. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A “transgene” is a gene that has been introduced into the genome by a transformation procedure.

“Coding sequence” refers to a DNA sequence that codes for a specific amino acid sequence. “Regulatory sequences” refer to nucleotide sequences located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, and polyadenylation recognition sequences.

“Promoter” refers to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3′ to a promoter sequence. The promoter sequence consists of proximal and more distal upstream elements, the latter elements often referred to as enhancers. Accordingly, an “enhancer” is a DNA sequence which can stimulate promoter activity and may be an innate element of the promoter or a heterologous element inserted to enhance the level or tissue-specificity of a promoter. Promoters may be derived in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental conditions. Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as “constitutive promoters”. New promoters of various types useful in plant cells are constantly being discovered; numerous examples may be found in the compilation by Okamuro and Goldberg, (1989) Biochemistry of Plants 15:1–82. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity.

The “translation leader sequence” refers to a DNA sequence located between the promoter sequence of a gene and the coding sequence. The translation leader sequence is present in the fully processed mRNA upstream of the translation start sequence. The translation leader sequence may affect processing of the primary transcript to mRNA, mRNA stability or translation efficiency. Examples of translation leader sequences have been described (Turner, R. and Foster, G. D. (1995) Molecular Biotechnology 3:225).

The “3′ non-coding sequences” refer to DNA sequences located downstream of a coding sequence and include polyadenylation recognition sequences and other sequences encoding regulatory signals capable of affecting mRNA processing or gene expression. The polyadenylation signal is usually characterized by affecting the addition of polyadenylic acid tracts to the 3′ end of the mRNA precursor. The use of different 3′ non-coding sequences is exemplified by Ingelbrecht et al., (1989) Plant Cell 1:671–680.

“RNA transcript” refers to the product resulting from RNA polymerase-catalyzed transcription of a DNA sequence. When the RNA transcript is a perfect complementary copy of the DNA sequence, it is referred to as the primary transcript or it may be a RNA sequence derived from posttranscriptional processing of the primary transcript and is referred to as the mature RNA. “Messenger RNA (mRNA)” refers to the RNA that is without introns and that can be translated into protein by the cell. “cDNA” refers to a double-stranded DNA that is complementary to and derived from mRNA. “Sense” RNA refers to RNA transcript that includes the mRNA and so can be translated into protein by the cell. “Antisense RNA” refers to a RNA transcript that is complementary to all or part of a target primary transcript or mRNA and that blocks the expression of a target gene (U.S. Pat. No. 5,107,065, incorporated herein by reference). The complementarity of an antisense RNA may be with any part of the specific gene transcript, i.e., at the 5′ non-coding sequence, 3′ non-coding sequence, introns, or the coding sequence. “Functional RNA” refers to sense RNA, antisense RNA, ribozyme RNA, or other RNA that may not be translated but yet has an effect on cellular processes.

The term “operably linked” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it is capable of affecting the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation.

The term “expression”, as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression may also refer to translation of mRNA into a polypeptide. “Antisense inhibition” refers to the production of antisense RNA transcripts capable of suppressing the expression of the target protein. “Overexpression” refers to the production of a gene product in transgenic organisms that exceeds levels of production in normal or non-transformed organisms. “Co-suppression” refers to the production of sense RNA transcripts capable of suppressing the expression of identical or substantially similar foreign or endogenous genes (U.S. Pat. No. 5,231,020, incorporated herein by reference).

“Altered levels” refers to the production of gene product(s) in transgenic organisms in amounts or proportions that differ from that of normal or non-transformed organisms.

“Mature” protein refers to a post-translationally processed polypeptide; i.e., one from which any pre- or propeptides present in the primary translation product have been removed. “Precursor” protein refers to the primary product of translation of mRNA; i.e., with pre- and propeptides still present. Pre- and propeptides may be but are not limited to intracellular localization signals.

A “chloroplast transit peptide” is an amino acid sequence which is translated in conjunction with a protein and directs the protein to the chloroplast or other plastid types present in the cell in which the protein is made. “Chloroplast transit sequence” refers to a nucleotide sequence that encodes a chloroplast transit peptide. A “signal peptide” is an amino acid sequence which is translated in conjunction with a protein and directs the protein to the secretory system (Chrispeels, J. J., (1991) Ann. Rev. Plant Phys. Plant Mol. Biol. 42:21–53). If the protein is to be directed to a vacuole, a vacuolar targeting signal (supra) can further be added, or if to the endoplasmic reticulum, an endoplasmic reticulum retention signal (supra) may be added. If the protein is to be directed to the nucleus, any signal peptide present should be removed and instead a nuclear localization signal included (Raikhel (1992) Plant Phys. 100:1627–1632).

“Transformation” refers to the transfer of a nucleic acid fragment into the genome of a host organism, resulting in genetically stable inheritance. Host organisms containing the transformed nucleic acid fragments are referred to as “transgenic” organisms. Examples of methods of plant transformation include Agrobacterium-mediated transformation (De Blaere et al. (1987) Meth. Enzymol. 143:277) and particle-accelerated or “gene gun” transformation technology (Klein et al. (1987) Nature (London) 327:70–73; U.S. Pat. No. 4,945,050, incorporated herein by reference).

Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described more fully in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory Press: Cold Spring Harbor, 1989 (hereinafter “Maniatis”).

Nucleic acid fragments encoding at least a portion of several tryptophan biosynthetic enzymes have been isolated and identified by comparison of random plant cDNA sequences to public databases containing nucleotide and protein sequences using the BLAST algorithms well known to those skilled in the art. Table 1 lists the proteins that are described herein, and the designation of the cDNA clones that comprise the nucleic acid fragments encoding these proteins.

TABLE 1 Tryptophan Biosynthetic Enzymes Enzyme Clone Plant Anthranilate phosphoribosyltransferase Contig of: Corn p0037.crwan82r p0022.cglne15r cbn10.pk0045.b8 p0109.cdadc66r cr1n.pk0145.g6 cco1.pk0002.f11 cs1.pk0056.d10 p0103.ciaag55r p0104.cabbg36r ssm.pk0059.f8 Soybean wre1.pk0003.b12 Wheat Indole-3-glycerol phosphate synthase cen3n.pk0147.h5 Corn Contig of: Corn p0128.cpicq73r p0041.crtav17rb p0002.cgevb40r p0091.cmarc86r cr1n.pk0121.b7 chp2.pk0003.c4 rl0n.pk0021.f11 Rice sdp2c.pk001.f3 Soybean wre1n.pk0075.b10 Wheat Phosphoribosylanthranilate isomerase Contig of: Corn cco1n.pk0030.b11 p0068.clsaa67r p0099.ctbai70r ceb1.pk0026.d2 Corn rsr9n.pk001.g2 Rice wr1.pk0127.e10 Wheat

The nucleic acid fragments of the instant invention may be used to isolate cDNAs and genes encoding homologous proteins from the same or other plant species. Isolation of homologous genes using sequence-dependent protocols is well known in the art. Examples of sequence-dependent protocols include, but are not limited to, methods of nucleic acid hybridization, and methods of DNA and RNA amplification as exemplified by various uses of nucleic acid amplification technologies (e.g., polymerase chain reaction, ligase chain reaction).

For example, genes encoding other anthranilate phosphoribosiltransferases, indole-3-glycerol phosphate synthases or phosphoribosylanthranilate isomerases, either as cDNAs or genomic DNAs, could be isolated directly by using all or a portion of the instant nucleic acid fragments as DNA hybridization probes to screen libraries from any desired plant employing methodology well known to those skilled in the art. Specific oligonucleotide probes based upon the instant nucleic acid sequences can be designed and synthesized by methods known in the art (Maniatis). Moreover, the entire sequences can be used directly to synthesize DNA probes by methods known to the skilled artisan such as random primer DNA labeling, nick translation, or end-labeling techniques, or RNA probes using available in vitro transcription systems. In addition, specific primers can be designed and used to amplify a part or all of the instant sequences. The resulting amplification products can be labeled directly during amplification reactions or labeled after amplification reactions, and used as probes to isolate full length cDNA or genomic fragments under conditions of appropriate stringency.

In addition, two short segments of the instant nucleic acid fragments may be used in polymerase chain reaction protocols to amplify longer nucleic acid fragments encoding homologous genes from DNA or RNA. The polymerase chain reaction may also be performed on a library of cloned nucleic acid fragments wherein the sequence of one primer is derived from the instant nucleic acid fragments, and the sequence of the other primer takes advantage of the presence of the polyadenylic acid tracts to the 3′ end of the mRNA precursor encoding plant genes. Alternatively, the second primer sequence may be based upon sequences derived from the cloning vector. For example, the skilled artisan can follow the RACE protocol (Frohman et al., (1988) Proc. Natl. Acad. Sci. USA 85:8998) to generate cDNAs by using PCR to amplify copies of the region between a single point in the transcript and the 3′ or 5′ end. Primers oriented in the 3′ and 5′ directions can be designed from the instant sequences. Using commercially available 3′ RACE or 5′ RACE systems (BRL), specific 3′ or 5′ cDNA fragments can be isolated (Ohara et al., (1989) Proc. Natl. Acad. Sci. USA 86:5673; Loh et al., (1989) Science 243:217). Products generated by the 3′ and 5′ RACE procedures can be combined to generate full-length cDNAs (Frohman, M. A. and Martin, G. R., (1989) Techniques 1:165).

Availability of the instant nucleotide and deduced amino acid sequences facilitates immunological screening of cDNA expression libraries. Synthetic peptides representing portions of the instant amino acid sequences may be synthesized. These peptides can be used to immunize animals to produce polyclonal or monoclonal antibodies with specificity for peptides or proteins comprising the amino acid sequences. These antibodies can be then be used to screen cDNA expression libraries to isolate full-length cDNA clones of interest (Lerner, R. A. (1984) Adv. Immunol. 36:1; Maniatis).

The nucleic acid fragments of the instant invention may be used to create transgenic plants in which the disclosed anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase are present at higher or lower levels than normal or in cell types or developmental stages in which they are not normally found. This would have the effect of altering the level of tryptophan biosynthesis in those cells. Overexpression of any one of these three enzymes should lead to the production of plants with higher levels of tryptophan, an essential amino acid. These enzymes catalyze a pathway not found in humans of higher animals making them ideal candidates for the discovery of herbicides.

Overexpression of the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase proteins of the instant invention may be accomplished by first constructing a chimeric gene in which the coding region is operably linked to a promoter capable of directing expression of a gene in the desired tissues at the desired stage of development. For reasons of convenience, the chimeric gene may comprise promoter sequences and translation leader sequences derived from the same genes. 3′ Non-coding sequences encoding transcription termination signals may also be provided. The instant chimeric gene may also comprise one or more introns in order to facilitate gene expression.

Plasmid vectors comprising the instant chimeric gene can then constructed. The choice of plasmid vector is dependent upon the method that will be used to transform host plants. The skilled artisan is well aware of the genetic elements that must be present on the plasmid vector in order to successfully transform, select and propagate host cells containing the chimeric gene. The skilled artisan will also recognize that different independent transformation events will result in different levels and patterns of expression (Jones et al., (1985) EMBO J. 4:2411–2418; De Almeida et al., (1989) Mol. Gen. Genetics 218:78–86), and thus that multiple events must be screened in order to obtain lines displaying the desired expression level and pattern. Such screening may be accomplished by Southern analysis of DNA, Northern analysis of mRNA expression, Western analysis of protein expression, or phenotypic analysis.

For some applications it may be useful to direct the instant tryptophan biosynthetic enzyme to different cellular compartments, or to facilitate its secretion from the cell. It is thus envisioned that the chimeric gene described above may be further supplemented by altering the coding sequence to encode anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase with appropriate intracellular targeting sequences such as transit sequences (Keegstra, K. (1989) Cell 56:247–253), signal sequences or sequences encoding endoplasmic reticulum localization (Chrispeels, J. J., (1991) Ann. Rev. Plant Phys. Plant Mol. Biol. 42:21–53), or nuclear localization signals (Raikhel, N. (1992) Plant Phys. 100:1627–1632) added and/or with targeting sequences that are already present removed. While the references cited give examples of each of these, the list is not exhaustive and more targeting signals of utility may be discovered in the future.

It may also be desirable to reduce or eliminate expression of genes encoding anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase in plants for some applications. In order to accomplish this, a chimeric gene designed for co-suppression of the instant tryptophan biosynthetic enzyme can be constructed by linking a gene or gene fragment encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase to plant promoter sequences. Alternatively, a chimeric gene designed to express antisense RNA for all or part of the instant nucleic acid fragment can be constructed by linking the gene or gene fragment in reverse orientation to plant promoter sequences. Either the co-suppression or antisense chimeric genes could be introduced into plants via transformation wherein expression of the corresponding endogenous genes are reduced or eliminated.

The instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase (or portions thereof) may be produced in heterologous host cells, particularly in the cells of microbial hosts, and can be used to prepare antibodies to the these proteins by methods well known to those skilled in the art. The antibodies are useful for detecting anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase in situ in cells or in vitro in cell extracts. Preferred heterologous host cells for production of the instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase are microbial hosts. Microbial expression systems and expression vectors containing regulatory sequences that direct high level expression of foreign proteins are well known to those skilled in the art. Any of these could be used to construct a chimeric gene for production of the instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase. This chimeric gene could then be introduced into appropriate microorganisms via transformation to provide high level expression of the encoded tryptophan biosynthetic enzyme. An example of a vector for high level expression of the instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase in a bacterial host is provided (Example 9).

Additionally, the instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase can be used as targets to facilitate design and/or identification of inhibitors of those enzymes that may be useful as herbicides. This is desirable because the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase and the phosphoribosylanthranilate isomerase described herein catalyze various steps in tryptophan biosynthesis. Accordingly, inhibition of the activity of one or more of the enzymes described herein could lead to inhibition plant growth. Thus, the instant anthranilate phosphoribosiltransferase, indole-3-glycerol phosphate synthase or phosphoribosylanthranilate isomerase could be appropriate for new herbicide discovery and design.

All or a substantial portion of the nucleic acid fragments of the instant invention may also be used as probes for genetically and physically mapping the genes that they are a part of, and as markers for traits linked to those genes. Such information may be useful in plant breeding in order to develop lines with desired phenotypes. For example, the instant nucleic acid fragments may be used as restriction fragment length polymorphism (RFLP) markers. Southern blots (Maniatis) of restriction-digested plant genomic DNA may be probed with the nucleic acid fragments of the instant invention. The resulting banding patterns may then be subjected to genetic analyses using computer programs such as MapMaker (Lander et at., (1987) Genomics 1:174–181) in order to construct a genetic map. In addition, the nucleic acid fragments of the instant invention may be used to probe Southern blots containing restriction endonuclease-treated genomic DNAs of a set of individuals representing parent and progeny of a defined genetic cross. Segregation of the DNA polymorphisms is noted and used to calculate the position of the instant nucleic acid sequence in the genetic map previously obtained using this population (Botstein, D. et al., (1980) Am. J. Hum. Genet. 32:314–331).

The production and use of plant gene-derived probes for use in genetic mapping is described in R. Bematzky, R. and Tanksley, S. D. (1986) Plant Mol. Biol. Reporter 4(1):37–41. Numerous publications describe genetic mapping of specific cDNA clones using the methodology outlined above or variations thereof. For example, F2 intercross populations, backcross populations, randomly mated populations, near isogenic lines, and other sets of individuals may be used for mapping. Such methodologies are well known to those skilled in the art.

Nucleic acid probes derived from the instant nucleic acid sequences may also be used for physical mapping (i.e., placement of sequences on physical maps; see Hoheisel, J. D., et al., In: Nonmammalian Genomic Analysis: A Practical Guide, Academic press 1996, pp. 319–346, and references cited therein).

In another embodiment, nucleic acid probes derived from the instant nucleic acid sequences may be used in direct fluorescence in situ hybridization (FISH) mapping (Trask, B. J. (1991) Trends Genet. 7:149–154). Although current methods of FISH mapping favor use of large clones (several to several hundred KB; see Laan, M. et al. (1995) Genome Research 5:13–20), improvements in sensitivity may allow performance of FISH mapping using shorter probes.

A variety of nucleic acid amplification-based methods of genetic and physical mapping may be carried out using the instant nucleic acid sequences. Examples include allele-specific amplification (Kazazian, H. H. (1989) J. Lab. Clin. Med. 114(2):95–96), polymorphism of PCR-amplified fragments (CAPS; Sheffield, V. C. et al. (1993) Genomics 16:325–332), allele-specific ligation (Landegren, U. et al. (1988) Science 241:1077–1080), nucleotide extension reactions (Sokolov, B. P. (1990) Nucleic Acid Res. 18:3671), Radiation Hybrid Mapping (Walter, M. A. et al. (1997) Nature Genetics 7:22–28) and Happy Mapping (Dear, P. H. and Cook, P. R. (1989) Nucleic Acid Res. 17:6795–6807). For these methods, the sequence of a nucleic acid fragment is used to design and produce primer pairs for use in the amplification reaction or in primer extension reactions. The design of such primers is well known to those skilled in the art. In methods employing PCR-based genetic mapping, it may be necessary to identify DNA sequence differences between the parents of the mapping cross in the region corresponding to the instant nucleic acid sequence. This, however, is generally not necessary for mapping methods.

Loss of function mutant phenotypes may be identified for the instant cDNA clones either by targeted gene disruption protocols or by identifying specific mutants for these genes contained in a maize population carrying mutations in all possible genes (Ballinger and Benzer, (1989) Proc. Natl. Acad. Sci USA 86:9402; Koes et al., (1995) Proc. Natl. Acad. Sci USA 92:8149; Bensen et al., (1995) Plant Cell 7:75). The latter approach may be accomplished in two ways. First, short segments of the instant nucleic acid fragments may be used in polymerase chain reaction protocols in conjunction with a mutation tag sequence primer on DNAs prepared from a population of plants in which Mutator transposons or some other mutation-causing DNA element has been introduced (see Bensen, supra). The amplification of a specific DNA fragment with these primers indicates the insertion of the mutation tag element in or near the plant gene encoding the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase. Alternatively, the instant nucleic acid fragment may be used as a hybridization probe against PCR amplification products generated from the mutation population using the mutation tag sequence primer in conjunction with an arbitrary genomic site primer, such as that for a restriction enzyme site-anchored synthetic adaptor. With either method, a plant containing a mutation in the endogenous gene encoding an anthranilate phosphoribosiltransferase, an indole-3-glycerol phosphate synthase or a phosphoribosylanthranilate isomerase can be identified and obtained. This mutant plant can then be used to determine or confirm the natural function of the anthranilate phosphoribosiltransferase, the indole-3-glycerol phosphate synthase or the phosphoribosylanthranilate isomerase gene product.

EXAMPLES

The present invention is further defined in the following Examples, in which all parts and percentages are by weight and degrees are Celsius, unless otherwise stated. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Example 1 Composition of cDNA Libraries; Isolation and Sequencing of cDNA Clones

cDNA libraries representing mRNAs from various corn, rice, soybean and wheat tissues were prepared. The characteristics of the libraries are described below.

TABLE 2 cDNA Libraries from Corn, Rice, Soybean and Wheat Library Tissue Clone cbn10 Corn Developing Kernel (Embryo and cbn10.pk0045.b8 Endosperm); 10 Days After Pollination cco1 Corn Cob of 67 Day Old Plants Grown in cco1.pk0002.f11 Green House cco1n Corn Cob of 67 Day Old Plants Grown in cco1n.pk0030.b11 Green House* ceb1 Corn Embryo 10 to 11 Days After ceb1.pk0026.d2 Pollination cen3n Corn Endosperm 20 Days After cen3n.pk0147.h5 Pollination* chp2 Corn 11 Day Old Leaf Treated 24 Hours chp2.pk0003.c4 With Herbicides** cr1n Corn Root From 7 Day Old Seedlings* cr1n.pk0121.b7 cr1n.pk0145.g6 csi1 Corn Silk cs1.pk0056.d10 p0002 Corn Tassel: Premeiotic, Early p0002.cgevb40r Uninucleate p0022 Mid Rib of the Middle ¾ of the 3rd Leaf p0022.cglne15r Blade From Corn Green Leaves Treated with 1 mg/ml Jasmonic Acid in 0.02% Tween 20–24 Hours Before Collection* p0037 Corn V5 Stage Roots Infested With Corn p0037.crwan82r Root Worm p0041 Corn Root Tips Smaller Than 5 mm in p0041.crtav17rb Length Four Days After Imbibition p0068 Corn Pericarp 28 Days After Pollination p0068.clsaa67r p0091 Corn Roots 2 and 3 Days After p0091.cmarc86r Germination, Pooled p0099 Corn Tassel: Apical Meristem; Floral p0099.ctbai70r Transition p0103 Corn Tassel Shoots (0.1–1.4 cm) p0103.ciaag55r p0104 Corn Roots V5 Corn Root Worm Infested p0104.cabbg36r p0109 Corn Leaves From Les9 Mutant***, Les9 p0109.cdadc66r Transition Zone and Les9 Mature Lesions, Pooled* p0128 Corn Primary and Secondary Immature p0128.cpicq73r Ear rl0n Rice 15 Day Old Leaf* rl0n.pk0021.f11 rsr9n Rice Leaf 15 Days After Germination rsr9n.pk001.g2 Harvested 2–72 Hours Following Infection With Magnaporta grisea (4360-R-62 and 4360-R-67)* sdp2c Soybean Developing Pods (6–7 mm) sdp2c.pk001.f3 ssm Soybean Shoot Meristem ssm.pk0059.f8 wr1 Wheat Root From 7 Day Old Seedling wr1.pk0127.e10 wre1 Wheat Root From 7 Day Old Etiolated wre1.pk0003.b12 Seedling wre1n Wheat Root From 7 Day Old Etiolated wre1n.pk0075.b10 Seedling* *These libraries were normalized essentially as described in U.S. Pat. No. 5,482,845 **Application of 2-[(2,4-dihydro-2,6,9-trimethyl[1]benzothiopyrano[4,3-c]pyrazol-8-yl)carbonyl]-1,3-cyclohexanedione S,S-dioxide (synthesis and methods of using this compound are described in WO 97/19087, incorporated herein by reference) and 2-[(2,3-dihydro-5,8-dimethylspiro[4H-1-benzothiopyran-4,2′-[1,3]dioxolan]-6-yl)carbonyl]-1,3-cyclohexanedione S,S-dioxide (also named 2-[(2,3-dihydro-5,8-dimethylspiro[4H-1-benzothiopyran-4,2′-[1,3]dioxolan]-6-yl)carbonyl]-3-hydroxy-2-cyclohexen-1-one S,S-dioxide; synthesis and methods of using this compound are described in WO 97/01550, incorporated herein by reference) ***Les9 mutants are described by Hoisington, D. (1986) Maize Genet Coop News Lett 60:50–51.

cDNA libraries were prepared in Uni-ZAP™ XR vectors according to the manufacturer's protocol (Stratagene Cloning Systems, La Jolla, Calif.). Conversion of the Uni-ZAP™ XR libraries into plasmid libraries was accomplished according to the protocol provided by Stratagene. Upon conversion, cDNA inserts were contained in the plasmid vector pBluescript. cDNA inserts from randomly picked bacterial colonies containing recombinant pBluescript plasmids were amplified via polymerase chain reaction using primers specific for vector sequences flanking the inserted cDNA sequences or plasmid DNA was prepared from cultured bacterial cells. Amplified insert DNAs or plasmid DNAs were sequenced in dye-primer sequencing reactions to generate partial cDNA sequences (expressed sequence tags or “ESTs”; see Adams, M. D. et al., (1991) Science 252:1651). The resulting ESTs were analyzed using a Perkin Elmer Model 377 fluorescent sequencer.

Example 2 Identification of cDNA Clones

ESTs encoding tryptophan biosynthetic enzymes were identified by conducting BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., (1993) J. Mol. Biol. 215:403–410) searches for similarity to sequences contained in the BLAST “nr” database (comprising all non-redundant GenBank CDS translations, sequences derived from the 3-dimensional structure Brookhaven Protein Data Bank, the last major release of the SWISS-PROT protein sequence database, EMBL, and DDBJ databases). The cDNA sequences obtained in Example 1 were analyzed for similarity to all publicly available DNA sequences contained in the “nr” database using the BLASTN algorithm provided by the National Center for Biotechnology Information (NCBI). The DNA sequences were translated in all reading frames and compared for similarity to all publicly available protein sequences contained in the “nr” database using the BLASTX algorithm (Gish, W. and States, D. J. (1993) Nature Genetics 3:266–272) provided by the NCBI. For convenience, the P-value (probability) of observing a match of a cDNA sequence to a sequence contained in the searched databases merely by chance as calculated by BLAST are reported herein as “pLog” values, which represent the negative of the logarithm of the reported P-value. Accordingly, the greater the pLog value, the greater the likelihood that the cDNA sequence and the BLAST “hit” represent homologous proteins.

Example 3 Characterization of cDNA Clones Encoding Anthranilate Phosphoribosyltransferase

The BLASTX search using the EST sequences from several corn, soybean and wheat clones revealed similarity of the proteins encoded by the cDNAs to anthranilate phosphoribosyltransferase from Arabidopsis thaliana. In the process of comparing the corn ESTs it was found that clones cbn10.pk0045.b8, cco1.pk0002.f11 and cs1.pk0056.d10 had overlapping regions of homology. Using this homology it was possible to align the ESTs and assemble a contig (a contig is an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence). The individual sequences were assembled into one contiguous nucleotide sequence encoding anthranilate phosphoribosyltransferase from corn. The database accession numbers and BLAST results for each of these ESTs and contigs are shown in Table 3:

TABLE 3 BLAST Results for Clones Encoding Polypeptides Homologous to Anthranilate phoshoribosyltransferase Database Clone Organism Accession No Blast Score pLog Contig of clones: A. thaliana GenBank U58942 54.15 cbn10.pk0045.b8 cco1.pk0002.f11 cs1.pk0056.d10 wre1.pk0003.b12 A. thaliana GenBank U58942 47.39 ssm.pk0059.f8 A. thaliana GenBank M96073 36.00

A longer corn contig was assembled with a portion of the cDNA inserts from clones p0037.crwan82r, p0022.cg1ne15r, cbn10.pk0045.b8, p0109.cdadc66r, cr1n.pk0145.g6, cco1.pk0002.f11, cs1.pk0056.d10, p0103.ciaag55r and p0104.cabbg36r and the sequence of the entire cDNA insert in clones ssm.pk0059.f8 and wre1.pk0003.b12 was determined. The BLASTX search using these sequences revealed similarity of the proteins encoded by the cDNAs to anthranilate phosphoribosyltransferase from Arabidopsis thaliana (NCBI General Identifier Nos. 1389768 and 401213). The BLAST results for each of these sequences are shown in Table 4:

TABLE 4 BLAST Results for Clones Encoding Polypeptides Homologous to Anthranilate Phosphoribosyltransferase BLAST pLog Score Clone 1389768 401213 Contig of: 159.0 159.0 p0037.crwan82r p0022.cglne15r cbn10.pk0045.b8 p0109.cdadc66r cr1n.pk0145.g6 cco1.pk0002.f11 cs1.pk0056.d10 p0103.ciaag55r p0104.cabbg36r ssm.pk0059.f8 95.70 96.22 wre1.pk0003.b12 96.10 96.00

The sequence of the contig assembled from a portion of the cDNA inserts from clones p0037.crwan82r, p0022.cg1ne15r, cbn10.pk0045.b8, p0109.cdadc66r, cr1n.pk0145.g6, cco1.pk0002.f11, cs1.pk0056.d10, p0103.ciaag55r and p0104.cabbg36r is shown in SEQ ID NO:1; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:2. The sequence of the entire cDNA insert from clone ssm.pk0059.f8 is shown in SEQ ID NO:3; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:4. The sequence of the entire cDNA insert from clone wre1.pk0003.b12 is shown in SEQ ID NO:5; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:6. BLAST scores and probabilities indicate that the instant nucleic acid fragments encode an almost entire conr anthranilate phosphoribosyltransferase and the C-terminal portions of soybean and wheat anthranilate phosphoribosyltransferase. These sequences represent the first corn, soybean and wheat sequences encoding anthranilate phosphoribosyltransferase.

Example 4 Characterization of Corn, Rice and Wheat cDNA Clones Encoding Indole-3-Glycerol Phosphate Synthase

The BLASTX search using the EST sequences from several corn, rice and wheat clones revealed similarity of the proteins encoded by the cDNAs to indole-3-glycerol phosphate synthase from Arabidopsis thaliana. In the process of comparing the corn ESTs it was found that clones cr1n.pk0121.b7 and cen3n.pk0047.d8 had overlapping regions of homology. A comparison of the wheat ESTs from clones wre1n.pk0075.b10 and wle1n.pk0039.d2 also had overlapping regions of homology. Using this homology it was possible to align the ESTs and assemble contigs (a contig is an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence). The individual sequences were assembled into unique contiguous nucleotide sequences encoding indole-3-glycerol phosphate synthase from corn and wheat. The database accession numbers and BLAST results for each of these ESTs and contigs are shown in Table 5:

TABLE 5 BLAST Results for Clones Encoding Polypeptides Homologous to Indole-3-Glycerol Phosphate Synthase Database Clone Organism Accession No Blast Score pLog Contig of clones: A. thaliana U18770 50.69 cr1n.pk0121.b7 cen3n.pk0047.d8 cen3n.pk0147.h5 A. thaliana U18770 19.15 rl0n.pk0021.f11 A. thaliana U18770 Contig of clones: A. thaliana U18770 56.39 wre1n.pk0075.b10 wle1n.pk0039.d2 wl1n.pk0112.b3 A.thaliana U18770 31.00

The sequence of the entire cDNA insert in clone cen3n.pk0147.h5 was determined and is shown in SEQ ID NO:7; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:8. A contig was assembled using the sequence of the entire cDNA insert in clones cr1n.pk0121.b7 and chp2.pk0003.c4 and the EST sequences in clones p0128.cpicq73r, p0041.crtav17rb, p0002.cgevb40r and p0091.cmarc86r. The sequence of this contig is shown in SEQ ID NO:9; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:10. The amino acid sequence set forth in SEQ ID NO:8 was evaluated by BLASTP, yielding a pLog value of 108.0 versus the Arabidopsis thaliana sequence. The sequence of the entire cDNA insert in clone r10n.pk0021.f11 was determined and is shown in SEQ ID NO:11; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:12. The sequence of the entire cDNA insert in clone wre1n.pk0075.b10 was determined and is shown in SEQ ID NO:15; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:16.

Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragments encode an entire corn indole-3-glycerol phosphate synthase and portions of corn, rice and wheat indole-3-glycerol phosphate synthase. These sequences represent the first corn, rice and wheat sequences encoding indole-3-glycerol phosphate synthase.

Example 5 Characterization and Expression in Microbial Cells of a Soybean cDNA Clone Encoding Indole-3-Glycerol Phosphate Synthase

The BLASTX search using the EST sequences from clone sdp2c.pk001.f3 revealed similarity of the proteins encoded by the cDNAs to indole-3-glycerol phosphate synthase precursor from Arabidopsis thaliana (NCBI General Identifier No. 1351303) with a pLog value of 12.15. The sequence of the entire cDNA insert in clone sdp2c.pk001.f3 was determined and is shown in SEQ ID NO:13; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:14. Bacteria containing clone sdp2c.pk001.f3 were obtained from the DuPont Genomics group and the plasmid isolated. The PCR cloning strategy outlined by Zeng G. (1998) Biotechniques 25:206–208 was used to clone the isolated fragment into plasmid pBX2 for expression of the protein in bacterial cells. The PCR primers used were:

SEQ ID NO:27 pIGSs-F1: 5′- TATG GAA GGT TTG GCT TCC CTC -3′ SEQ ID NO:28 pIGSs-F2: 5′- TG GAA GGT TTG GCT TCC CTC -3′ SEQ ID NO:29 pIGSs-M1: 5′- TATG GTT CTG AAA ATT AAG GAG TGG -3′ SEQ ID NO:30 pIGSs-M2: 5′- TG GTT CTG AAA ATT AAG GAG TGG -3′ SEQ ID NO:31 pSK-B1: 5′- GATCC ATA GGG GCA ATT GGG TAC CGG -3′ SEQ ID NO:32 pSK-B2: 5′- C ATA GGG GCA ATT GGG TAC CGG -3′

Primers with an F designation were used to isolate clones with a chloroplast transit peptide sequence (CTP) while those designated with an M were used to isolate clones for the mature protein (no CTP). Primers with pSKB designation are generic primers for use with the pBluescript SK plasmid. PCR reactions were performed with the Expand High Fidelity PCR System of Boehringer Mannheim. Four different PCR reactions were performed with the following primer combinations:

Reaction 1: pIGSs-F2 with pSKB1; the DNA obtained will be referred to as F2B1;

Reaction 2: pIGSs-F1 with pSKB2; the DNA obtained will be referred to as F1B2;

Reaction 3: pIGS-M2 with pSKB1; the DNA obtained will be referred to as M2B1;

Reaction 4: pIGS-M1 with pSKB2; the DNA obtained will be referred to as M1B2.

PCR was performed with a touchdown program: 94° C. for 3 minutes; 94° C. for 15 sec, 66° C. for 30 sec (to 57° C. in 10 cycles), 72° C. for 1 min 30 sec; 94° C. for 15 sec, 57° C. for 30 sec, 72° C. for 1 min 30 sec (25 cycles); 72° C. 7 min; hold at 4° C. until use. PCR products were separated using a 1% agarose TAE gel and DNA in bands was purified using the Qiaex II agarose gel extraction protocol from Qiagen. The final DNA product was eluted in 40 μL of 10 mM Tris-HCl, Ph 8.5. M1B2 was pooled with M2B1 and concentrated to 25 μL total volume using a speed vac. F1B2 was pooled with F2B1 and concentrated to 25 μL total volume using a speed vac. The concentrated mixtures were heated to 95° C. and allowed to cool down to room temperature over about 20 minutes. Ligations were set up with 1 μL of Nde I/Bam H1-digested pBX2, 5 μL water, 2 μL T4 DNA ligase 5×buffer (Gibco/BRL), 1 μL T4 DNA ligase(Gibco/BRL) and either 1 μL of M1B2+M2B1 (for ligation 1) or 1 μL F1B2+F2B1 (for ligation 2). Ligations proceeded at room temperature for 4 hours. The ligation mix was transformed into DH5alpha competent E. coli using standard heat shock methods, plated onto LB/carbenicillin plates and grown overnight at 37° C. Plasmids with the correct inserts were identified by double digest of miniprep DNA using Nde I and Bam HI. Plasmid containing the full-length gene is pBX2-IGPS-3 and plasmid containing the gene for the mature protein (no putative chloroplast transit peptide sequence) is pBX2-IGPS-4. Two separate colonies for each construct were used for midi plasmid preps. The identity of the resulting DNA was confirmed by DNA sequencing.

The plasmid DNAs were also transformed into BL21(DE3) pLysS cells using a rapid transformation protocol (Pope B. and Kent H. M. (1996) Nucleic Acids Research 24:536–537) and bacteria grown on LB/carbenicillin/chloramphenicol plates overnight at 37° C. Colonies were selected and streaked for isolation. Well-isolated colonies were used to produce frozen glycerol stocks.

For protein production, LB/carbenicillin/chloramphenicol plates were streaked with the appropriate glycerol stock and grown overnight at 37° C. A single well-isolated colony was used to inoculate 1 liter of liquid LB (100 μg/mL carbenicillin, 34 μg/mL chloroamphenicol). Bacteria was grown at room temperature (26° C.) until reaching an optical density of between 0.4 and 1.0 at 600 nm. IPTG was added to a final concentration of 0.5 mM and bacteria subsequently harvested by centrifugation 4 hours later. Bacterial pellets were stored at −80° C. until use. Pellets were thawed in extraction buffer which consisted of 300 mM NaCl, 10 mM imidazole, 10% glycerol, 10 mM β-mercaptoethanol, 0.1% Triton X-100, 20 mM HEPES, 1 mM AEBSF (protease inhibitor), 1 μg/mL leupeptin (protease inhibitor), 1 μg/ml antipain (protease inhibitor), pH 7.9. All subsequent steps were performed on ice or in a cold room. DNA in the extracts was sheared using a probe sonicator and then extracts clarified through centrifugation. 0.5 mL of Qiagen Ni-NTA agarose was added to each supernatant and then extracts placed on an end-over-end mixer for 1 hour. Ni-NTA agarose was transferred to a column and resin washed several times with extraction buffer. IGPS protein was eluted with 1 mL of elution buffer (extraction buffer minus protease inhibitors and containing 250 mM imidazole instead of 10 mM imidazole).

Enzyme assays were performed on a SLM Aminco 8000 spectrofluorometer with all slits set to 16 nm, excitation 280 nm, emission 350 nm, gain 100, HV 250. The assay was based on Hankins, C. N. et al. (1975) Anal Biochem 69:510–517. Assay buffer was pH 7.9 and contained 50 mM EPPS, 1 mM DTT, 10% glycerol, 5 mM EDTA, and 100 μg/L BSA. Using 8 μM of 1-(2-carboxyphenylamino)-1-deoxyribulose-5-phosphate (CdRP) substrate prepared according to Kirschner K et al. (1987) Methods Enzymol 142:386–397 the following activities were measured:

TABLE 6 Activity of Cloned Arabidopsis and Soybean Indole-3-Glycerol Phosphate Synthase Protein Fluorescent units/μg protein* IGPS1 (Arabidopsis)** 1.48 ± 0.12 IGPS3*** 3.09 ± 0.22 IGPS4**** 4.36 ± 0.25 *mean ± spread of two determinations **Arabidopsis indole-3-glycerol phosphate synthase gene (GenBank Accession No. U18770) was placed in the pBX2 vector essentially by the method described above. Enzymatically active recombinant protein expression has not been previously reported for this gene. ***Protein purified from cells harboring the pBX2-IGPS-3 plasmid. ****Protein purified from cells harboring the pBX2-IGPS-4 plasmid.

The pH optimum for all three proteins was 7.9. Km for IGPS 1 is 3 μM and IGPS4 is 2 μM. Fluorescent units are not converted to amount of product since a sample of indole-3-glycerolphosphate is not available to create a standard curve. The data indicate that the instant nucleic acid fragments encode an enzyme with Indole-3-Glycerol Phosphate Synthase activity. Availability of the instant nucleic acid fragments and assays for enzyme activity will facilitate screening of chemical compounds for inhibitor activity and will lead to the discovery and development of herbicidal compounds.

FIGS. 2A and 2B presents an alignment of the amino acid sequences set forth in SEQ ID NOs: 10 and 14 and the Arabidopsis thaliana indole-3-glycerol phosphate synthase sequence (SEQ ID NO:27). The amino acid sequence set forth in SEQ ID NO: 10 54.3% similar to the Arabidopsis thaliana sequence while the amino acid sequence set forth in SEQ ID NO:14 is 67.9% similar to the Arabidopsis thaliana sequence. Sequence alignments and percent similarity calculations were performed using the Megalign program of the LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences was performed using the Clustal method of alignment (Higgins, D. G. and Sharp, P. M. (1989) CABJOS. 5:151–153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10).

Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragment encodes an entire enzymatically active soybean indole-3-glycerol phosphate synthase. This sequence represents the first soybean sequence encoding indole-3-glycerol phosphate synthase.

Example 6 Characterization of cDNA Clones Encoding Phosphoribosylanthranilate Isomerase

The BLASTX search using the EST sequences from several corn and wheat clones revealed similarity of the proteins encoded by the cDNAs to phosphoribosylanthranilate isomerase from Arabidopsis thaliana. In the process of comparing the corn ESTs it was found that clones ceb1.pk0026.d2 and csi1n.pk0048.f11 had overlapping regions of homology. A comparison of the wheat ESTs from clones wr1.pk0127.e10 and wr1.pk0102.b8 also had overlapping regions of homology. Using this homology it was possible to align the ESTs and assemble contigs (a contig is an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence). The individual sequences were assembled into unique contiguous nucleotide sequences encoding anthranilate phosphoribosyltransferase from corn and wheat. The database accession numbers and BLAST results for each of these ESTs and contigs are shown in Table 7:

TABLE 7 BLAST Results for Clones Encoding Polypeptides Homologous to Phosphoribosylanthranilate Isomerase Database Clone Organism Accession No Blast Score pLog Contig of clones: A. thaliana GenBank U18969 14.39 ceb1.pk0026.d2 csi1n.pk0048.f11 Contig of clones: A. thaliana GenBank U34757 7.22 wr1.pk0127.e10 wr1.pk0102.b8

The BLASTX search using the sequences of the contig assembled from clones cco1n.pk0030.b11, p0068.c1saa67r and p0099.ctbai70r, using the EST sequences from clone rsr9n.pk001.g2 and using the sequence of the entire cDNA insert in clones ceb1.pk0026.d2 and wr1.pk0127.e10 revealed similarity of the proteins encoded by the cDNAs to Phosphoribosylanthranilate Isomerase from Arabidopsis thaliana (NCBI General Identifier No. 619749). The BLAST results for each of these sequences are shown in Table 8:

TABLE 8 BLAST Results for Clones Encoding Polypeptides Homologous to Phosphoribosylanthranilate Synthase BLAST pLog Score Clone 619749 cco1n.pk0030.b11 37.00 p0068.clsaa67r p0099.ctbai70r ceb1.pk0026.d2 82.70 rsr9n.pk001.g2 45.70 wr1.pk0127.e10 80.52

The sequence of the contig assembled from a portion of the cDNA insert from clones cco1n.pk0030.b11, p0068.c1saa67r and p0099.ctbai70r is shown in SEQ ID NO:17; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:18. The sequence of the entire cDNA insert in clone ceb1.pk0026.d2 was determined and is shown in SEQ ID NO:19; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:20. The nucleotide sequence set forth in SEQ ID NO:19 includes the sequence from a portion of the cDNA insert in clone csi1n.pk0048.f11. The amino acid sequence set forth in SEQ ID NO:20 was evaluated by BLASTP, yielding a pLog value of 74.0 versus the Arabidopsis thaliana sequence. The sequence of a portion of the cDNA insert from clone rsr9n.pk001.g2 is shown in SEQ ID NO:21; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:22. The sequence of the entire cDNA insert in clone wr1.pk0127.e10 was determined and is shown in SEQ ID NO:23; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:24. The nucleotide sequence set forth in SEQ ID NO:23 includes the sequence from a portion of the cDNA insert in clonewr1.pk0102.b8. The amino acid sequence set forth in SEQ ID NO:24 was evaluated by BLASTP, yielding a pLog value of 72.00 versus the Arabidopsis thaliana sequence.

FIGS. 3A and B presents an alignment of the amino acid sequences set forth in SEQ ID NOs:20 and 24 and the Arabidopsis thaliana phosphoribosylanthranilate isomerase sequence. Sequence alignments and percent similarity calculations were performed using the Megalign program of the LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences was performed using the Clustal method of alignment (Higgins, D. G. and Sharp, P. M. (1989) CABIOS. 5:151–153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10).

Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragments encode entire or nearly entire corn and wheat phosphribosylanthranilate isomerase and portions of corn and rice phosphoribosylanthranilate isomerase. These sequences represent the first corn, rice and wheat sequences encoding phosphoribosylanthranilate isomerase.

Example 7 Expression of Chimeric Genes in Monocot Cells

A chimeric gene comprising a cDNA encoding a tryptophan biosynthetic enzyme in sense orientation with respect to the maize 27 kD zein promoter that is located 5′ to the cDNA fragment, and the 10 kD zein 3′ end that is located 3′ to the cDNA fragment, can be constructed. The cDNA fragment of this gene may be generated by polymerase chain reaction (PCR) of the cDNA clone using appropriate oligonucleotide primers. Cloning sites (NcoI or SmaI) can be incorporated into the oligonucleotides to provide proper orientation of the DNA fragment when inserted into the digested vector pML103 as described below. Amplification is then performed in a standard PCR. The amplified DNA is then digested with restriction enzymes NcoI and SmaI and fractionated on an agarose gel. The appropriate band can be isolated from the gel and combined with a 4.9 kb NcoI-SmaI fragment of the plasmid pML103. Plasmid pML103 has been deposited under the terms of the Budapest Treaty at ATCC (American Type Culture Collection, 10801 University Blvd., Manassas, Va. 20110-2209), and bears accession number ATCC 97366. The DNA segment from pML103 contains a 1.05 kb SalI-NcoI promoter fragment of the maize 27 kD zein gene and a 0.96 kb SmaI-SalI fragment from the 3′ end of the maize 10 kD zein gene in the vector pGem9Zf(+) (Promega). Vector and insert DNA can be ligated at 15° C. overnight, essentially as described (Maniatis). The ligated DNA may then be used to transform E. coli XL1-Blue (Epicurian Coli XL-1 Blue™; Stratagene). Bacterial transformants can be screened by restriction enzyme digestion of plasmid DNA and limited nucleotide sequence analysis using the dideoxy chain termination method (Sequenase™ DNA Sequencing Kit; U. S. Biochemical). The resulting plasmid construct would comprise a chimeric gene encoding, in the 5′ to 3′ direction, the maize 27 kD zein promoter, a cDNA fragment encoding a tryptophan biosynthetic enzyme, and the 10 kD zein 3′ region.

The chimeric gene described above can then be introduced into corn cells by the following procedure. Immature corn embryos can be dissected from developing caryopses derived from crosses of the inbred corn lines H99 and LH132. The embryos are isolated 10 to 11 days after pollination when they are 1.0 to 1.5 mm long. The embryos are then placed with the axis-side facing down and in contact with agarose-solidified N6 medium (Chu et al., (1975) Sci. Sin. Peking 18:659–668). The embryos are kept in the dark at 27° C. Friable embryogenic callus consisting of undifferentiated masses of cells with somatic proembryoids and embryoids borne on suspensor structures proliferates from the scutellum of these immature embryos. The embryogenic callus isolated from the primary explant can be cultured on N6 medium and sub-cultured on this medium every 2 to 3 weeks.

The plasmid, p35S/Ac (obtained from Dr. Peter Eckes, Hoechst Ag, Frankfurt, Germany) may be used in transformation experiments in order to provide for a selectable marker. This plasmid contains the Pat gene (see European Patent Publication 0 242 236) which encodes phosphinothricin acetyl transferase (PAT). The enzyme PAT confers resistance to herbicidal glutamine synthetase inhibitors such as phosphinothricin. The pat gene in p35S/Ac is under the control of the 35S promoter from Cauliflower Mosaic Virus (Odell et al. (1985) Nature 313:810–812) and the 3′ region of the nopaline synthase gene from the T-DNA of the Ti plasmid of Agrobacterium tumefaciens.

The particle bombardment method (Klein et al., (1987) Nature 327:70–73) may be used to transfer genes to the callus culture cells. According to this method, gold particles (1 μm in diameter) are coated with DNA using the following technique. Ten μg of plasmid DNAs are added to 50 μL of a suspension of gold particles (60 mg per mL). Calcium chloride (50 μL of a 2.5 M solution) and spermidine free base (20 μL of a 1.0 M solution) are added to the particles. The suspension is vortexed during the addition of these solutions. After 10 minutes, the tubes are briefly centrifuged (5 sec at 15,000 rpm) and the supernatant removed. The particles are resuspended in 200 μL of absolute ethanol, centrifuged again and the supernatant removed. The ethanol rinse is performed again and the particles resuspended in a final volume of 30 μL of ethanol. An aliquot (5 μL) of the DNA-coated gold particles can be placed in the center of a Kapton™ flying disc (Bio-Rad Labs). The particles are then accelerated into the corn tissue with a Biolistic™ PDS-1000/He (Bio-Rad Instruments, Hercules Calif.), using a helium pressure of 1000 psi, a gap distance of 0.5 cm and a flying distance of 1.0 cm.

For bombardment, the embryogenic tissue is placed on filter paper over agarose-solidified N6 medium. The tissue is arranged as a thin lawn and covered a circular area of about 5 cm in diameter. The petri dish containing the tissue can be placed in the chamber of the PDS-1000/He approximately 8 cm from the stopping screen. The air in the chamber is then evacuated to a vacuum of 28 inches of Hg. The macrocarrier is accelerated with a helium shock wave using a rupture membrane that bursts when the He pressure in the shock tube reaches 1000 psi.

Seven days after bombardment the tissue can be transferred to N6 medium that contains gluphosinate (2 mg per liter) and lacks casein or proline. The tissue continues to grow slowly on this medium. After an additional 2 weeks the tissue can be transferred to fresh N6 medium containing gluphosinate. After 6 weeks, areas of about 1 cm in diameter of actively growing callus can be identified on some of the plates containing the glufosinate-supplemented medium. These calli may continue to grow when sub-cultured on the selective medium.

Plants can be regenerated from the transgenic callus by first transferring clusters of tissue to N6 medium supplemented with 0.2 mg per liter of 2,4-D. After two weeks the tissue can be transferred to regeneration medium (From et al., (1990) Bio/Technology 8:833–839).

Example 8 Expression of Chimeric Genes in Dicot Cells

A seed-specific expression cassette composed of the promoter and transcription terminator from the gene encoding the β subunit of the seed storage protein phaseolin from the bean Phaseolus vulgaris (Doyle et al. (1986) J. Biol. Chem. 261:9228–9238) can be used for expression of the instant tryptophan biosynthetic enzymes in transformed soybean. The phaseolin cassette includes about 500 nucleotides upstream (5′) from the translation initiation codon and about 1650 nucleotides downstream (3′) from the translation stop codon of phaseolin. Between the 5′ and 3′ regions are the unique restriction endonuclease sites Nco I (which includes the ATG translation initiation codon), Sma I, Kpn I and Xba I. The entire cassette is flanked by Hind III sites.

The cDNA fragment of this gene may be generated by polymerase chain reaction (PCR) of the cDNA clone using appropriate oligonucleotide primers. Cloning sites can be incorporated into the oligonucleotides to provide proper orientation of the DNA fragment when inserted into the expression vector. Amplification is then performed as described above, and the isolated fragment is inserted into a pUC18 vector carrying the seed expression cassette.

Soybean embroys may then be transformed with the expression vector comprising sequences encoding a tryptophan biosynthetic enzyme. To induce somatic embryos, cotyledons, 3–5 mm in length dissected from surface sterilized, immature seeds of the soybean cultivar A2872, can be cultured in the light or dark at 26° C. on an appropriate agar medium for 6–10 weeks. Somatic embryos which produce secondary embryos are then excised and placed into a suitable liquid medium. After repeated selection for clusters of somatic embryos which multiplied as early, globular staged embryos, the suspensions are maintained as described below.

Soybean embryogenic suspension cultures can maintained in 35 mL liquid media on a rotary shaker, 150 rpm, at 26° C. with florescent lights on a 16:8 hour day/night schedule. Cultures are subcultured every two weeks by inoculating approximately 35 mg of tissue into 35 mL of liquid medium.

Soybean embryogenic suspension cultures may then be transformed by the method of particle gun bombardment (Kline et al. (1987) Nature (London) 327:70, U.S. Pat. No. 4,945,050). A DuPont Biolistic™ PDS1000/HE instrument (helium retrofit) can be used for these transformations.

A selectable marker gene which can be used to facilitate soybean transformation is a chimeric gene composed of the 35S promoter from Cauliflower Mosaic Virus (Odell et al. (1985) Nature 313:810–812), the hygromycin phosphotransferase gene from plasmid pJR225 (from E. coli; Gritz et al.(1983) Gene 25:179–188) and the 3′ region of the nopaline synthase gene from the T-DNA of the Ti plasmid of Agrobacterium tumefaciens. The seed expression cassette comprising the phaseolin 5′ region, the fragment encoding the tryptophan biosynthetic enzyme and the phaseolin 3′ region can be isolated as a restriction fragment. This fragment can then be inserted into a unique restriction site of the vector carrying the marker gene.

To 50 μL of a 60 mg/mL 1 μm gold particle suspension is added (in order): 5 μL DNA (1 μg/μL), 20 μl spermidine (0.1 M), and 50 μL CaCl₂ (2.5 M). The particle preparation is then agitated for three minutes, spun in a microfuge for 10 seconds and the supernatant removed. The DNA-coated particles are then washed once in 400 μL 70% ethanol and resuspended in 40 μL of anhydrous ethanol. The DNA/particle suspension can be sonicated three times for one second each. Five μL of the DNA-coated gold particles are then loaded on each macro carrier disk.

Approximately 300–400 mg of a two-week-old suspension culture is placed in an empty 60×15 mm petri dish and the residual liquid removed from the tissue with a pipette. For each transformation experiment, approximately 5–10 plates of tissue are normally bombarded. Membrane rupture pressure is set at 1100 psi and the chamber is evacuated to a vacuum of 28 inches mercury. The tissue is placed approximately 3.5 inches away from the retaining screen and bombarded three times. Following bombardment, the tissue can be divided in half and placed back into liquid and cultured as described above.

Five to seven days post bombardment, the liquid media may be exchanged with fresh media, and eleven to twelve days post bombardment with fresh media containing 50 mg/mL hygromycin. This selective media can be refreshed weekly. Seven to eight weeks post bombardment, green, transformed tissue may be observed growing from untransformed, necrotic embryogenic clusters. Isolated green tissue is removed and inoculated into individual flasks to generate new, clonally propagated, transformed embryogenic suspension cultures. Each new line may be treated as an independent transformation event. These suspensions can then be subcultured and maintained as clusters of immature embryos or regenerated into whole plants by maturation and germination of individual somatic embryos.

Example 9 Expression of Chimeric Genes in Microbial Cells

The cDNAs encoding the instant tryptophan biosynthetic enzyme can be inserted into the T7 E. coli expression vector pBT430. This vector is a derivative of pET-3a (Rosenberg et al. (1987) Gene 56:125–135) which employs the bacteriophage T7 RNA polymerase/T7 promoter system. Plasmid pBT430 was constructed by first destroying the EcoR I and Hind III sites in pET-3a at their original positions. An oligonucleotide adaptor containing EcoR I and Hind III sites was inserted at the BamH I site of pET-3a. This created pET-3aM with additional unique cloning sites for insertion of genes into the expression vector. Then, the Nde I site at the position of translation initiation was converted to an Nco I site using oligonucleotide-directed mutagenesis. The DNA sequence of pET-3aM in this region, 5′-CATATGG, was converted to 5′-CCCATGG in pBT430.

Plasmid DNA containing a cDNA may be appropriately digested to release a nucleic acid fragment encoding the protein. This fragment may then be purified on a 1% NuSieve GTG™ low melting agarose gel (FMC). Buffer and agarose contain 10 μg/ml ethidium bromide for visualization of the DNA fragment. The fragment can then be purified from the agarose gel by digestion with GELase™ (Epicentre Technologies) according to the manufacturer's instructions, ethanol precipitated, dried and resuspended in 20 μL of water. Appropriate oligonucleotide adapters may be ligated to the fragment using T4 DNA ligase (New England Biolabs, Beverly, Mass.). The fragment containing the ligated adapters can be purified from the excess adapters using low melting agarose as described above. The vector pBT430 is digested, dephosphorylated with alkaline phosphatase (NEB) and deproteinized with phenol/chloroform as described above. The prepared vector pBT430 and fragment can then be ligated at 16° C. for 15 hours followed by transformation into DH5 electrocompetent cells (GIBCO BRL). Transformants can be selected on agar plates containing LB media and 100 μg/mL ampicillin. Transformants containing the gene encoding the tryptophan biosynthetic enzyme are then screened for the correct orientation with respect to the T7 promoter by restriction enzyme analysis.

For high level expression, a plasmid clone with the cDNA insert in the correct orientation relative to the T7 promoter can be transformed into E. coli strain BL21(DE3) (Studier et al. (1986) J. Mol. Biol. 189:113–130). Cultures are grown in LB medium containing ampicillin (100 mg/L) at 25° C. At an optical density at 600 nm of approximately 1, IPTG (isopropylthio-β-galactoside, the inducer) can be added to a final concentration of 0.4 mM and incubation can be continued for 3 h at 25°. Cells are then harvested by centrifugation and re-suspended in 50 μL of 50 mM Tris-HCl at pH 8.0 containing 0.1 mM DTT and 0.2 mM phenyl methylsulfonyl fluoride. A small amount of 1 mm glass beads can be added and the mixture sonicated 3 times for about 5 seconds each time with a microprobe sonicator. The mixture is centrifuged and the protein concentration of the supernatant determined. One μg of protein from the soluble fraction of the culture can be separated by SDS-polyacrylamide gel electrophoresis. Gels can be observed for protein bands migrating at the expected molecular weight.

Example 10 Evaluating Compounds for Their Ability to Inhibit the Activity of Tryptophan Biosynthetic Enzymes

The tryptophan biosynthetic enzymes described herein may be produced using any number of methods known to those skilled in the art. Such methods include, but are not limited to, expression in bacteria as described in Example 9, or expression in eukaryotic cell culture, in planta, and using viral expression systems in suitably infected organisms or cell lines. The instant tryptophan biosynthetic enzymes may be expressed either as mature forms of the proteins as observed in vivo or as fusion proteins by covalent attachment to a variety of enzymes, proteins or affinity tags. Common fusion protein partners include glutathione S-transferase (“GST”), thioredoxin (“Trx”), maltose binding protein, and C- and/or N-terminal hexahistidine polypeptide (“(His)₆”). The fusion proteins may be engineered with a protease recognition site at the fusion point so that fusion partners can be separated by protease digestion to yield intact mature enzyme. Examples of such proteases include thrombin, enterokinase and factor Xa. However, any protease can be used which specifically cleaves the peptide connecting the fusion protein and the enzyme.

Purification of the instant tryptophan biosynthetic enzymes, if desired, may utilize any number of separation technologies familiar to those skilled in the art of protein purification. Examples of such methods include, but are not limited to, homogenization, filtration, centrifugation, heat denaturation, ammonium sulfate precipitation, desalting, pH precipitation, ion exchange chromatography, hydrophobic interaction chromatography and affinity chromatography, wherein the affinity ligand represents a substrate, substrate analog or inhibitor. When the tryptophan biosynthetic enzymes are expressed as fusion proteins, the purification protocol may include the use of an affinity resin which is specific for the fusion protein tag attached to the expressed enzyme or an affinity resin containing ligands which are specific for the enzyme. For example, a tryptophan biosynthetic enzyme may be expressed as a fusion protein coupled to the C-terminus of thioredoxin. In addition, a (His)₆ peptide may be engineered into the N-terminus of the fused thioredoxin moiety to afford additional opportunities for affinity purification. Other suitable affinity resins could be synthesized by linking the appropriate ligands to any suitable resin such as Sepharose-4B. In an alternate embodiment, a thioredoxin fusion protein may be eluted using dithiothreitol; however, elution may be accomplished using other reagents which interact to displace the thioredoxin from the resin. These reagents include β-mercaptoethanol or other reduced thiol. The eluted fusion protein may be subjected to further purification by traditional means as stated above, if desired. Proteolytic cleavage of the thioredoxin fusion protein and the enzyme may be accomplished after the fusion protein is purified or while the protein is still bound to the ThioBond™ affinity resin or other resin.

Crude, partially purified or purified enzyme, either alone or as a fusion protein, may be utilized in assays for the evaluation of compounds for their ability to inhibit enzymatic activation of the tryptophan biosynthetic enzymes disclosed herein. Assays may be conducted under well known experimental conditions which permit optimal enzymatic activity. For example, assays for anthranilate phophoribosyltransferase are presented by Hommel, U. et al. (1989) Eur J Biochem 180:33–40. Assays for indole-3-glycerol phosphate synthase are presented by Hankins, C. N. et al. (1975) Anal Biochem 69:510–517). Assays for phosphoribosylanthranilate isomerase are presented by Last, R. L. and Fink, G. R. (1988) Science 240:305–310.

APPENDIX A      Clustal V alignment of the corn and wheat phosphoribosyl-anthranilate isomerase amino acid sequences of the present invention (SEQ ID NO:20 and SEQ ID NO:24, respectively) with the phosphoribosylanthranilate isomerase amino acid sequences of Arabidopsis thaliana, Saccharomyces cerevisiae, and E. coli found in the Li et al. alignment (AtPAI1, ATPAI2, AtPAI3, yeast, and E. coli, respectively). The conserved domains found in the Li et al. article are shown boxed, and the invariant and conserved residues from Sterner et al. are indicated above the alignment. The invariant residues are in upper case letters and the conserved residues are in lower case. The program uses dashes to maximize the alignment.   1 MLLASSTRRYEQFPLA--RNNGLPRFSRVKMSC--LGTNQSNHHSDTVRSS---SPSCGD SEQ ID 20   1 MATAFSTKQPLRVATP--TNKWRPRLPLIKMQY-----SSNKRASASISLP---SSAEGV SEQ ID 24   1 MSTGISTDLHVHFGALNFSKTYKSGLSNRTVSFSRVGYAQNRKLSCSVSNTENVAPKDDE AtPAI1   1 MSTGISTDLHVHFGALNFSKTYKSGLSNRTVSFSRVGYAQNRKLSCSVSNTENVAPKDDE AtPAI2   1 MSTGISSDLHLHPRALNFSKTSKSGLSNRKVSFSSVGYAQNRKLSCSVSSTENVAPKDDD AtPAI3   1 N----------------------------------------------------------- E coli   1 MSV------------INFT----------------------------------------- yeast     -----------+-------+---------+---------+---------+---------+                10      20        30        40        50        60             K CG             ga   G  54 TRKVHPVVKMCGITSARDAEMAVKAGAELIGMILWPNSKRSVSLLEAKEISRVVQSYGAE SEQ ID 20  51 ERN-EPIVKMCGITSARDAEFAAKAGAKLIGMILWPKSKRSVQRSEAKEISRVAKSYGAE SEQ ID 24  61 RGKDRPLVKMCGITSARDAAMAVEAGADFIGMIIWPHSKRSISLSVAKDISKVAREGGAK AtPAI1  61 RGKDRPLVKMCGITSARDAAMAVEAGADFIGMIIWPHSKRSISLSVAKDISKVAREGGAK AtPAI2  61 RGKDRPLVKMCGITSARDAAMAVEAGADFIGMIIWPHSKRSISLSVAKDISQVAREGGAK AtPAI3   2 --------KVCGLTRGQDAKAAYDAGAIYGGLIFVATSPRCVNVEQAQEV---------M E coli   8 -GSSGPLVKVCGLQSTEAAECALDSDADLLGIICVPNRKRTIDPVIARKISSLVKAYKNS yeast     ---------+---------+---------+---------+---------+---------+              70        80        90        100       110       120             V VFV                     QlHg 114 S------ VGVFV DDNEETILRVSDSCDLNF VQLHG DESRALVHTLSKNNRIVYVLNADDD SEQ ID 20 110 A------ VGVFV DDDEETILRVADSCNLQL IQLHG DSSRALVPALAKNNRIVYVLNADAD SEQ ID 24 121 P------ VGVFV EDDENTILRAADSSDLEL VQLHG NGSRAAFSRLVRKRRVIYVLNANQD AtPAI1 121 P------ VGVFV EDDDNTILRAADSSDLEL VQLHG NGSRAAFSRLVRKRRVIYVLNANQD AtPAI2 121 P------ VGVFV EDDENTILRAADSSDLEL VQLHG NSSRAAFSRLVRERKVIYVLNANED AtPAI3  45 AAAPLQY VGVFR NHDIADVVDKAKVLSLVA VQLHG NEEQLYIDTL--REALPAHVAIWKA E coli  67 SGTPKYL VGVFR NQPKEDVLALVNDYGIDI VQLHG DESWQEYQEFLGLPVIKRLVFPKDC yeast     ------- --+-- -------+---------+ ----- ----+---------+---------+               130        140       150       160       170       180                         D     G G                          laGGl 168 GKLINIPDIE-YEL-DWYLVDSAK GGSGKGFNWQKFQMPSVKSKNG-----WL LAGGL HA SEQ ID 20 164 GKLINSPPSEEYDI-DWFLVDSAE GGSGKGFNWDNFRMPSVKSKNG-----WL LAGGL HA SEQ ID 24 175 GKLLNEVPEEDCHLADWILVDSAT GGSGHGFNWAQFKLPSVRSRNG-----WL LAGGI NP AtPAI1 175 GKLLNEVPEEDCHLADWILVDSAT GGSGHGFNWAQFKLPSVRSRNG-----WL LAGGI NP AtPAI2 175 GKLLNVVPEEDGHLADWILVDSAT GGSGKGFNWAQFKLPSVRSRNG-----WL LAGGI NP AtPAI3 103 LSVGETLPAREFQHVDKYVLDNGQ GGSGQRFDWSLL--------NGQTLGNVL LAGGL GA E coli 127 NILLSAASQKPHSFIP--LFDSEA GGTGELLDWNSISDWVGRQESPESL-HFM LAGGL TP yeast     ---------+---------+---- -----+---------+---------+--- ----- -+              190       200       210       220       230       240      N              D  sgv     G   d 221 DNVCEAFSALKPD GVDVSSGI CGRDGIRKDADRINSFISNVKSLN-----FLS SEQ ID 20 218 DNVCQAASALKPN GVDVSSGI CSPDGISKDPKRISSFMRSVQSLSSRRGLYLDAPGLL SEQ ID 24 230 TNVSEALSILQPD GIDVSSGI CGTDGIQKDKSKISSFITAVRSVH-----Y AtPAI1 230 TNVSEALSILQPD GIDVSSGI CGTDGIQKDKSKISSFITAVRSVH-----Y AtPAI2 230 TNVSEALSILQPD GIDVSSGI CGIDGIQKDKSKISSFITAVRSVH-----Y AtPAI3 155 DNCVEA-AQTGCA GLDFNSAV ESQPGI-KDARLLASVFQTLRA-------Y E coli 184 ENVGDALRLNGVI GVDVSGGV -ETNGV-KDSNKIANFVKNAKK yeast     ---------+--- ------+- --------+---------+---------+--------              250        260        270       280       290 

1. An isolated polynucleotide comprising: (a) a nucleotide sequence encoding a polypeptide having phosphoribosylanthranilate isomerase activity, wherein the amino acid sequence of the polypeptide and the amino acid sequence of SEQ ID NO:20 have at least 90% identity, or (b) the complement of the nucleotide sequence.
 2. The polynucleotide of claim 1, wherein the amino acid sequence of the polypeptide and the amino acid sequence of SEQ ID NO:20 have at least 95% identity.
 3. The polynucleotide of claim 1, wherein the nucleotide sequence comprises the nucleotide sequence of SEQ ID NO:19.
 4. The polynucleotide of claim 1, wherein the amino acid sequence of the polypeptide comprises the amino acid sequence of SEQ ID NO:20.
 5. A vector comprising the polynucleotide of claim
 1. 6. A chimeric gene comprising the polynucleotide of claim 1 operably linked to a regulatory sequence.
 7. A method for transforming a cell comprising transforming a cell with the polynucleotide of claim
 1. 8. An isolated cell comprising the chimeric gene of claim
 6. 9. A method for producing a plant comprising transforming a plant cell with the polynucleotide of claim 1 and regenerating a plant from the transformed plant cell.
 10. A plant comprising the chimeric gene of claim
 6. 11. A seed comprising the chimeric gene of claim
 6. 12. A method for producing a polypeptide encoded by the chimeric gene of claim 6 comprising: (a) transforming a cell with the chimeric gene; and (b) culturing said cell in a medium to promote expression of the polypeptide encoded by the chimeric gene. 